1. Field of the Invention
The present invention relates to semiconductor laser diodes, and more particularly to semiconductor laser diodes with the double hetero junction structure.
2. Description of the Prior Art
Referring to FIG. 1, there is illustrated a semiconductor laser diode with a conventional double hetero junction structure of the inner stripe type. As shown in FIG. 1, the semiconductor laser diode has a semiconductor substrate 11, a buffer layer 12, a clad layer 13 of a first conductivity type, an active layer 14, a clad layer 15 of a second conductivity type, a current limitation layer 16 and a cap layer 17, all layers being sequentially grown over the substrate 11. The semiconductor laser diode also has a metal electrode 18 of the first conductivity type and a metal electrode 19 of the second conductivity type grown beneath and over the substrate 11, respectively.
In this structure, current introduced in the laser diode through the second conductivity type metal electrode 19 is limited by the current limitation layer 16 while passing through the cap layer 17. As a result, the current is introduced in the second conductivity type clad layer 15 at a current density. The current is radiative recombined in the active layer 14 and then introduced in the first conductivity type metal electrode 18 via the first conductivity type clad layer 13, the buffer layer 12 and the substrate 11.
The active layer 14 has a smaller energy band gap and a larger refractive index than those of the clad layers 13 and 15 disposed over and beneath the active layer 14. With such characteristics, electrons and holes are gathered in the active layer. Also, light radiated is concentrated by the active layer 14, because of the refractive index difference between the active layer 14 and each of the clad layers 13 and 15, so that the oscillation initiating current becomes lowered.
The buffer layer 12 serves to buffer crystal defects present in the substrate 11, so as to obtain a better crystal layer.
When current is injected into the above-mentioned semiconductor laser diode, however, a flow of current feels a barrier caused by the large energy band gap difference between the cap layer 17 and the second conductivity type clad layer 15 or between the substrate 11 (or the buffer layer) and the first conductivity type clad layer 13. As a result, a resistance component against the flow of current is generated, resulting in a generation of heat. This results in a degradation in electrooptical characteristics of devices.
In FIG. 4, there is illustrated an energy band structure of the conventional laser diode.
Referring to FIG. 4, it can be found that a high barrier of the spike type occurs due to discontinuous energy levels, in particular, at boundaries between the cap layer 17 and each of the clad layers 13 and 15 and between the substrate 11 and each of the clad layers 13 and 15. Due to the barrier, flowing of electrons and holes are limited, thereby causing parasitic radiative resistance components to be generated.
As a result, the operating current becomes higher, thereby resulting in a radiation of heat and a degradation in reliability. Moreover, an undesirable phenomenon such as a variation in wavelength of emitted light may occur due to the radiation of heat.